I have no idea if the Japanese regularly assign any significance to Halloween, or to the time-venerated tradition of dressing up in silly costumes and harassing your neighbors for candy.
...But for the sake of a story, let's pretend they do!
"Hey Sousuke, it's been a long time since we've done anything together, right?"
"We're in near-constant proximity to each other, my occasional absences notwithstanding."
Kaname was well aware of the cause for his 'occasional absences', and didn't begrudge him for them. She sometimes worried herself to the point of illness, but remained faithful in his inevitable return from whatever battlefield he'd been sent to.
"Yeah, I know, I meant... you know... together together. You and me. As a couple."
Sousuke's expression didn't much change, but his slight nod was indication enough that he understood. ... or at least, that he thought he understood.
"So, since Halloween is coming up in about a week, I thought we could go trick-or-treating together."
The mercenary bodyguard looked at Kaname. "...Trick...or-treating?" he repeated. Coming from his mouth like that, Kaname felt that the words sounded quite foreign and out-of-place.
Suddenly recalling the Valentine's Day debacle, Kaname decided to give her socially clueless boyfriend some context.
But then, an idea suddenly occurred to her: if Sousuke couldn't adapt to peaceful social situations, then why not do the inverse of that? Why not try to adapt a normal situation to Sousuke?
It seemed an eternal pattern that wherever Sousuke tread, chaos followed in his footsteps. Usually accompanied by a symphony of gunfire and explosives. It wasn't really Sousuke's fault that he saw terrorist conspiracies where there were none, and couldn't fit in in peaceful Japan. Inevitably, Sousuke would interpret something innocuous as a terrorist plot, and in an effort to protect her, he'd just start slinging bullets.
That pattern had played out countless times before, and it certainly would do so again.
But if Kaname could spin trick-or-treating into a military context for Sousuke's benefit, construct a scenario where Sousuke could relax somewhat... could their destiny of chaos be averted?
Yes it can, Kaname decided. This time, Sousuke and I WILL beat the meyhem curve!
Growing confident in her new idea, Kaname worked to hide the mischievous grin from her face.
"Yeah, trick-or-treating. It's the code name for a military training exercise centered around children and civilians. And it happens in a lot of countries across the entire world, every Halloween. I'm actually kind of surprised you haven't heard of it."
Sousuke, for the first time Kaname had known him, looked genuinely surprised. Or at least, she thought he looked surprised. His expression didn't really change... at all... but she was still pretty sure.
"A training exercise... centered around children and civilians?"
"Um, yeah! You dress up in a costume and go from house-to-house, and say 'trick or treat!', and hold out a bag for the host to put some candy in. The objective is..." here, Kaname stumbled for a moment, having not quite managed to get that far. However, a poster for the drama club's next production - Dracula, incidentally - inspired her. "The objective is to introduce participants to the idea of pretending to be something different from their usual self. You know, acting out a cover. It's sort of like taking acting classes, but only one night a year and the curriculum is centered around young children. "
Sousuke nodded, seemingly accepting this logic. It seemed peculiar in the extreme that several countries would use the exact same propaganda method to instill children with an interest in espionage... but if this were so, then surely it must be a testament to the program's effectiveness.
"I see."
Kaname smiled, then, and Sousuke allowed himself to relax, slightly.
Maybe he'd give this "trick-or-treating" thing a shot...